Did you have a look at the masteries of the champs? There are some masteries which grant you a bonus to your turnmeter...in this case it might trigger for your Trunda and give her a boost, placing her before Madame...

Lore of Steel mastery ?

Hrmm...they both have Lore of Steel - but that should already be taken into account.  As noted above - 90% of the time Madam Seris goes first as I expect - and this is prior to the opponent doing anything.  So while it is possible that a mastery might kick it off? I don't see how it would do it so randomly before either of them or the opponent has a turn?

lore of steel applies to the set bonus, so if trunda has a/or more speed sets then seris that might be it. 

edit: or trunda with speeds set and seris with perception sets would still creat a mismatch with that lore of steel, as the benefit on speed sets would be bigger

Lore of steel is not the mastery I am talking about. 

Arcane Celerity would be the critical one:

Has a 30% chance of increasing the Turn meter by 10% when a debuff cast by this Champion is removed or expires...

True, there are a few masteries that could explain it, but not without the opponents doing anything... Another thing that could explain it is if somehow you were putting HP burns on the opponents, considering Trunda's passive giving her extra speed for each HP burn on an opponent. But neither Arbi nor Siphi puts HP burn. 

One more possibility: does this happen when you're facing Valkyries, perhaps? Her passive would decrease TM on Serris for putting up her shield, which happens even before she gets her turn, and which could be enough to make Trunda faster. 

I am going with passive of opponent as the answer here

that sounds the most plausible didnt think of that

Can you show us Trunda and Serris' stats/masteries? Kinda leaning towards L9753's comment too with Valkyries possibly being the main cause for this. 

Siphi will increase speed but that is for all allies unless an enemy champ is faster than Serris and removes buffs from Serris or reduces speed for serris.

i

Try sending only Trunda and Serris and see who goes first.

Trunda does have an ability which adds speed but that will come into play later

i

 It has to be an enemy champ fooling around, why dont you try to increase the resistance of serris to higher than trunda's and whatever the enemy champ is doing , serris should be able to resist it

It is also important to consider the team you are facing, were you facing Valkyrie for example? In that case when you give a boost to your team it is possible that Valkyrie's mastery reduced your serris turn meter but trunda resisted it

Thank you all - I'll take note to see if this specifically occurs with Valkyrie as an opponent.  Off the top of my head, I'd say no - but I can't say for sure.

Trunda's resist is much higher than Madam Seris (226 vs 130) so ..an opponent's passive could definitely be coming into play.

i